arm64: defconfig: Enable Virtio RNG driver as built in

arm64: defconfig: Enable Virtio RNG driver as built in

Modern versions of FVP_Base_RevC-2xAEMvA contain a Virtio RNG device,
use of which can dramatically speed up the time taken for Linux to
initialize its CRNG, from 10s of seconds in some cases, to instant. This
improves the debug cycle significantly.

However, there are 2 barriers to getting people to use it. The first is
that the defconfig doesn't have the required driver. The second is that
the device is disabled in the device tree (for back-compat since older
versions of the FVP don't have the device). Here we solve the first
issue.

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>